Myoclonus describes
sudden and uncontrollable jerks of a muscle or a group of muscles. The
word, "myoclonus", comes from "myo" meaning muscle and "clonus" meaning
twitch. It can happen at rest (negative myoclonus) or with movement (positive
myoclonus). Myoclonus can occur infrequently or every few seconds and can
range in severity from a minor irritation to a disabling symptom that can
make walking or other activities very difficult.

Some myoclonic jerks
are quite normal - hiccoughs (hippcups) and the twitches as you fall asleep
are both examples of myoclonus. However, myoclonus is often associated
with neurological or biochemical conditions including multiple
sclerosis, Parkinson's disease, stroke, spinal cord injury, kidney
or liver failure and lipid storage disease. Other forms occur in the absence
of any other disease. Myoclonus in one or both legs during the night is
known as Noctural Myoclonus or Restless Leg Syndrome (RLS) and can also occur
in the absence of any other disease. Myoclonic jerks are often seen during
epileptic siezures.

Although it can be
caused by damage to peripheral nerves,
most forms of myoclonus are caused by problems in the central
nervous system. The precise mechanisms unlying it are not fully understood
and it is possible that the different forms may have different causes.
Some people speculate that it results from a chemical imbalance in neurotransmitters
causing nerves to become over-excitable. Another theory is that nerve signals
from the brain, that would normally suppress these primitive reflexes,
may be prevented from getting through by nerve damage.

Clonazepam, Piracetam,
Sodium valproate, Phenytoin and Primidone and several other tranquilising
or anti-epilepsy drugs are often used to treat myoclonus.